This invention relates generally to non-reciprocal devices and more particularly to miniature microwave circulators and isolators.
As is known in the art, so-called microwave monolithic integrated circuits include active and passive devices which are formed using semiconductor integration circuit techniques to provide various types of microwave circuits. One such particular application of this technology is in so-called transmit/receive modules (transceiver modules) for use in phased array antennas. In transceiver modules active devices such as field effect transistors are combined with passive devices such as capacitors, resistors, inductive elements, and the like to form various microwave functions such as amplifiers and switches. It is desirable in transceiver modules to have one or more elements which can act as circulators and thus can be used to steer electromagnetic energy through the transceiver module.
Although non-reciprocal microwave components such as circulators and isolators have been known for many years, no convenient approach is available for combining the circulator on a common substrate which supports the integrated circuits. Generally, these non-reciprocal components are fabricated separately on different substrates than the semiconductor components or the transceiver module. The non-reciprocal component such as the circulator are fabricated either on a ferrite substrate or on a dielectric substrate that has a ferrite insert. Both methods of fabrication have a disadvantage of requiring relatively long connecting transmission lines between integrated circuits and the circulator with the attendant transmission line loss. This hybrid-type of approach is also relatively labor intensive and, therefore, costly.
Circulators which are intended for use with microwave integrated circuits are usually designed for coupling by means of microstrip transmission lines. An example of a circulator known in the art is a microstrip circulator formed on a ferrite substrate by patterning a top surface metal plane. The metal pattern comprises a circular section, described as the junction resonator and three radial line sections emerging at equally spaced points on the resonator's circumference. These lines are of designed width and length and serve to impedance match the junction to a desired level at the circulator's microstrip terminals.
Another example of a circulator which is well known in the art includes a dielectric substrate having a ground plane conductor disposed over a first surface thereof and a ferrite inserted within a hole provided in the dielectric substrate. In this particular design, strip conductors of microstrip transmission lines are connected to a central metal disc that is approximately the same diameter as the ferrite disc. Critical design parameters for this type of circulator are the radius of the metal disc (R) and the coupling angle which is defined as half the angle subtended by each microstrip at the perimeter of the central disc. Circulator design of the type described above have been demonstrated to have the capacity for very large bandwidths.
Another type of circulator which is also well known in the art and is also based on a microstrip transmission medium uses a ferrite member having coupling structure fabricated on the surface of the ferrite disc. As reported by R. H. Knerr, et al. IEEE Transactions, MTT-18, page 1100-1108, December 1970, three mask levels were required to fabricate the coupling structure by photolithographic techniques. Two mask levels were required for the metalization, and one mask level was required for the dielectric layer which separates the layers of metalization. This circulator design achieves a significant size reduction compared to the aforementioned microstrip circulator approaches.
If one were to adapt this latter approach to monolithic integrated circuits based upon prior techniques, one would have to drill a hole in a dielectric or semiconductor substrate and insert the member with the interwoven coupling structure into the hole.
Each of the microstrip designs described above requires a hole to be drilled into the dielectric substrate to receive the ferrite disc. This requirement presents severe manufacturing limitations when the circulators are fabricated on brittle substrates such as gallium arsenide, particularly when the intent is to integrate the circuit with monolithic microwave integrated circuits provided on the gallium arsenide substrate. Accordingly, the requirement of a hole in these types of circulators makes such circulators generally not suitable for integration with semiconductor elements on brittle substrates like gallium arsenide.
Other problems with each of the above approaches include the difficulty of fabricating the circular ferrite member, particularly when the ferrite member carries or supports an interwoven coupling structure.
These fabrication difficulties can be avoided by using a substrate without a hole and placing the ferrite disc with a coupling structure on top of the substrate, and thus otherwise retaining structure as described above. The problem with this approach is that this technique does not lead to circulators with useful performance, since the electromagnetic field does not penetrate sufficiently into the ferrite but rather remains concentrated in the dielectric substrate.
Accordingly, it would be desirable to provide a circulator and other magnetic non-reciprocal devices which can be readily integrated with monolithic microwave integrated circuits, and further which are relatively easy to fabricate, but which provide circulators having useful practical performance.